I've been working in a Dynamic filter class for EF Core queries using Expression trees, everything looks good, filter is working, I can pass a filter collection and it works, but when I look at the SQL sentence, it is querying the whole table and applying the filter on the resulting collection, here is my class...
public static class QueryExpressionBuilder
{
private static readonly MethodInfo ContainsMethod = typeof(string).GetMethod("Contains", new[] { typeof(string) });
private static readonly MethodInfo StartsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
private static readonly MethodInfo EndsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
#region DynamicWhere
/// <summary>Where expression generator.</summary>
/// <typeparam name="T"></typeparam>
/// <param name="filters">The filters.</param>
/// <returns></returns>
public static Expression<Func<T, bool>> GetExpression<T>(IList<Filter> filters)
{
if (filters.Count == 0)
return null;
ParameterExpression param = Expression.Parameter(typeof(T), "t");
Expression exp = null;
if (filters.Count == 1)
exp = GetExpression(param, filters[0]);
else if (filters.Count == 2)
exp = GetExpression<T>(param, filters[0], filters[1]);
else
{
while (filters.Count > 0)
{
var f1 = filters[0];
var f2 = filters[1];
if (exp == null)
exp = GetExpression<T>(param, filters[0], filters[1]);
else
exp = Expression.AndAlso(exp, GetExpression<T>(param, filters[0], filters[1]));
filters.Remove(f1);
filters.Remove(f2);
if (filters.Count == 1)
{
exp = Expression.AndAlso(exp, GetExpression(param, filters[0]));
filters.RemoveAt(0);
}
}
}
return Expression.Lambda<Func<T, bool>>(exp, param);
}
/// <summary>Comparision operator expression generator.</summary>
/// <param name="param">The parameter.</param>
/// <param name="filter">The filter.</param>
/// <returns></returns>
private static Expression GetExpression(ParameterExpression param, Filter filter)
{
MemberExpression member = Expression.Property(param, filter.PropertyName);
var type = member.Type;
ConstantExpression constant;
switch (type.Name)
{
case "Int32":
constant = Expression.Constant(Convert.ToInt32(filter.Value));
break;
case "String":
default:
constant = Expression.Constant(filter.Value);
break;
}
// ConstantExpression constant = Expression.Constant(filter.Value);
switch (filter.Operation)
{
case Op.Equals:
return Expression.Equal(member, constant);
case Op.GreaterThan:
return Expression.GreaterThan(member, constant);
case Op.GreaterThanOrEqual:
return Expression.GreaterThanOrEqual(member, constant);
case Op.LessThan:
return Expression.LessThan(member, constant);
case Op.LessThanOrEqual:
return Expression.LessThanOrEqual(member, constant);
case Op.Contains:
return Expression.Call(member, ContainsMethod, constant);
case Op.StartsWith:
return Expression.Call(member, StartsWithMethod, constant);
case Op.EndsWith:
return Expression.Call(member, EndsWithMethod, constant);
}
return null;
}
/// <summary>And logic connector expression generator.</summary>
/// <typeparam name="T"></typeparam>
/// <param name="param">The parameter.</param>
/// <param name="filter1">The filter1.</param>
/// <param name="filter2">The filter2.</param>
/// <returns></returns>
private static BinaryExpression GetExpression<T>(ParameterExpression param, Filter filter1, Filter filter2)
{
var bin1 = GetExpression(param, filter1);
var bin2 = GetExpression(param, filter2);
return Expression.AndAlso(bin1, bin2);
}
#endregion
}
}
To call this class I do something like this :
var whereDeleg = QueryExpressionBuilder.GetExpression<Tax>(filters).Compile();
var myList = _dbContext.MyEntity.Where(whereDeleg).ToList();
The filters parameter i'm passing is a collection of this class :
public class Filter
{
public string PropertyName { get; set; }
public Op Operation { get; set; }
public object Value { get; set; }
}
I'd appreciate any help.
The main problem is not the class, but the way you use it:
var whereDeleg = QueryExpressionBuilder.GetExpression<Tax>(filters).Compile();
var myList = _dbContext.MyEntity.Where(whereDeleg).ToList();
You are taking Expression<Func<T, bool>> from your method, but then the Complie() call converts it to Func<T, bool>. So although _dbContext.MyEntity is IQueryable<T>, there is no IQueryable<T> extension method Where taking Func<T, bool> (all they take Expression<Func<T, bool>>). But since IQueryable<T> inherits (hence is a) IEnumerable<T>, the compiler finds and uses the Where extension method for IEnumerable<T> (defined in Enumerable class).
This makes the Where (and all following methods if any) to execute client side after executing and materializing the query before Where (in your case - the whole table).
The difference between IQueryable<T> and IEnumerable<T> is covered by Returning IEnumerable<T> vs. IQueryable<T>. All you need is to make sure you always call IQueryable<T> extension methods instead of the IEnumerable<T> methods with the same name and similarly looking arguments by using Expression<Func<...>> instead of Func<...>.
With all that being said, you should use your method result directly without calling Compile:
var predicate = QueryExpressionBuilder.GetExpression<Tax>(filters);
var myList = _dbContext.MyEntity.Where(predicate).ToList();
or just
var myList = _dbContext.MyEntity.Where(QueryExpressionBuilder.GetExpression<Tax>(filters)).ToList();
Or even better, add the following custom extension method to the QueryExpressionBuilder class:
public static IQueryable<T> Where<T>(this IQueryable<T> source, IList<Filter> filters)
{
var predicate = GetExpression<T>(filters);
return predicate != null ? source.Where(predicate) : source;
}
to be able to use simply (and minimize the chance of making mistakes):
var myList = _dbContext.MyEntity.Where(filters).ToList();
Side note: The main expression builder method implementation is overcomplicated and also destroys the passed input filters list. It can be simplified as follows (which doesn't have the aforementioned defect):
public static Expression<Func<T, bool>> GetExpression<T>(IEnumerable<Filter> filters)
{
var param = Expression.Parameter(typeof(T), "t");
var body = filters
.Select(filter => GetExpression(param, filter))
.DefaultIfEmpty()
.Aggregate(Expression.AndAlso);
return body != null ? Expression.Lambda<Func<T, bool>>(body, param) : null;
}